net: sfp: initialize i2c_block_size at adapter configure time

sfp->i2c_block_size is only assigned in sfp_sm_mod_probe(), which runs
from the state machine timer after SFP_F_PRESENT has been set. Between
those two points, sfp_module_eeprom() (the ethtool -m callback) gates
only on SFP_F_PRESENT and can be entered with i2c_block_size still at
its kzalloc'd value of 0.

On a pure-I2C adapter, sfp_i2c_read() then issues an i2c_transfer()
with msgs[1].len = 0 inside a loop that subtracts this_len from len
each iteration; on adapters that succeed a zero-length read the loop
never advances, spinning while holding rtnl_lock.

This was previously addressed by initializing i2c_block_size in
sfp_alloc() (commit 813c2dd786), but the initialization was dropped
when i2c_block_size was split from i2c_max_block_size.

Initialize sfp->i2c_block_size from sfp->i2c_max_block_size in
sfp_i2c_configure(), so the field is valid as soon as the adapter is
known. sfp_sm_mod_probe() still reassigns it on each module insertion
to recover from a per-module clamp to 1 (sfp_id_needs_byte_io).

Fixes: 7662abf4db ("net: phy: sfp: Add support for SMBus module access")
Cc: stable@vger.kernel.org
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://patch.msgid.link/20260528205242.971410-2-jelonek.jonas@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/sfp.c

@@ -822,6 +822,7 @@ static int sfp_i2c_configure(struct sfp *sfp, struct i2c_adapter *i2c)
 		return -EINVAL;
 	}
 
+	sfp->i2c_block_size = sfp->i2c_max_block_size;
 	return 0;
 }