open.2: ERRORS: EBUSY

An EBUSY error case is mentioned in DESCRIPTIOn; add it to ERRORS
also.

See also https://bugzilla.kernel.org/show_bug.cgi?id=209109

Reported-by: henrik@optoscale.no
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/open.2

@@ -1014,6 +1014,14 @@ and
 in
 .BR proc (5).
 .TP
+.B EBUSY
+.B O_EXCL
+was specified in
+.I flags
+and
+.I pathname
+refers to a block device that is in use by the system (e.g., it is mounted).
+.TP
 .B EDQUOT
 Where
 .B O_CREAT